VISA or "Virtual Instrument Software Architecture"

is a standard library used to communicate to instruments. It's incorporated into Keysight Technologies "IO Suite" and also in National Instruments "Max". The library can be used by graphical and textual languages.
The VISA standard includes specifications for communication with instruments over Test and Measurement-specific I/O interfaces such as GPIB, VXI, PXI, AXIe .... There are also specifications for T&M-specific protocols (computing) over PC-standard I/O, such as VXI-11 (over TCPIP) and USBTMC (over USB).
The current standard, "VISA Specification 5.8", is maintained by the IVI Foundation. 
The complexity (communication protocol) of the different interfaces like: USB, LAN, PXI ... is solved in this library. So you can concentrate on the actual measurement task.